1XFS Self Describing Metadata
2----------------------------
34Introduction
5------------
67The largest scalability problem facing XFS is not one of algorithmic
8scalability, but of verification of the filesystem structure. Scalabilty of the
9structures and indexes on disk and the algorithms for iterating them are
10adequate for supporting PB scale filesystems with billions of inodes, however it
11is this very scalability that causes the verification problem.
1213Almost all metadata on XFS is dynamically allocated. The only fixed location
14metadata is the allocation group headers (SB, AGF, AGFL and AGI), while all
15other metadata structures need to be discovered by walking the filesystem
16structure in different ways. While this is already done by userspace tools for
17validating and repairing the structure, there are limits to what they can
18verify, and this in turn limits the supportable size of an XFS filesystem.
1920For example, it is entirely possible to manually use xfs_db and a bit of
21scripting to analyse the structure of a 100TB filesystem when trying to
22determine the root cause of a corruption problem, but it is still mainly a
23manual task of verifying that things like single bit errors or misplaced writes
24weren't the ultimate cause of a corruption event. It may take a few hours to a
25few days to perform such forensic analysis, so for at this scale root cause
26analysis is entirely possible.
2728However, if we scale the filesystem up to 1PB, we now have 10x as much metadata
29to analyse and so that analysis blows out towards weeks/months of forensic work.
30Most of the analysis work is slow and tedious, so as the amount of analysis goes
31up, the more likely that the cause will be lost in the noise. Hence the primary
32concern for supporting PB scale filesystems is minimising the time and effort
33required for basic forensic analysis of the filesystem structure.
343536Self Describing Metadata
37------------------------
3839One of the problems with the current metadata format is that apart from the
40magic number in the metadata block, we have no other way of identifying what it
41is supposed to be. We can't even identify if it is the right place. Put simply,
42you can't look at a single metadata block in isolation and say "yes, it is
43supposed to be there and the contents are valid".
4445Hence most of the time spent on forensic analysis is spent doing basic
46verification of metadata values, looking for values that are in range (and hence
47not detected by automated verification checks) but are not correct. Finding and
48understanding how things like cross linked block lists (e.g. sibling
49pointers in a btree end up with loops in them) are the key to understanding what
50went wrong, but it is impossible to tell what order the blocks were linked into
51each other or written to disk after the fact.
5253Hence we need to record more information into the metadata to allow us to
54quickly determine if the metadata is intact and can be ignored for the purpose
55of analysis. We can't protect against every possible type of error, but we can
56ensure that common types of errors are easily detectable. Hence the concept of
57self describing metadata.
5859The first, fundamental requirement of self describing metadata is that the
60metadata object contains some form of unique identifier in a well known
61location. This allows us to identify the expected contents of the block and
62hence parse and verify the metadata object. IF we can't independently identify
63the type of metadata in the object, then the metadata doesn't describe itself
64very well at all!
6566Luckily, almost all XFS metadata has magic numbers embedded already - only the
67AGFL, remote symlinks and remote attribute blocks do not contain identifying
68magic numbers. Hence we can change the on-disk format of all these objects to
69add more identifying information and detect this simply by changing the magic
70numbers in the metadata objects. That is, if it has the current magic number,
71the metadata isn't self identifying. If it contains a new magic number, it is
72self identifying and we can do much more expansive automated verification of the
73metadata object at runtime, during forensic analysis or repair.
7475As a primary concern, self describing metadata needs some form of overall
76integrity checking. We cannot trust the metadata if we cannot verify that it has
77not been changed as a result of external influences. Hence we need some form of
78integrity check, and this is done by adding CRC32c validation to the metadata
79block. If we can verify the block contains the metadata it was intended to
80contain, a large amount of the manual verification work can be skipped.
8182CRC32c was selected as metadata cannot be more than 64k in length in XFS and
83hence a 32 bit CRC is more than sufficient to detect multi-bit errors in
84metadata blocks. CRC32c is also now hardware accelerated on common CPUs so it is
85fast. So while CRC32c is not the strongest of possible integrity checks that
86could be used, it is more than sufficient for our needs and has relatively
87little overhead. Adding support for larger integrity fields and/or algorithms
88does really provide any extra value over CRC32c, but it does add a lot of
89complexity and so there is no provision for changing the integrity checking
90mechanism.
9192Self describing metadata needs to contain enough information so that the
93metadata block can be verified as being in the correct place without needing to
94look at any other metadata. This means it needs to contain location information.
95Just adding a block number to the metadata is not sufficient to protect against
96mis-directed writes - a write might be misdirected to the wrong LUN and so be
97written to the "correct block" of the wrong filesystem. Hence location
98information must contain a filesystem identifier as well as a block number.
99100Another key information point in forensic analysis is knowing who the metadata
101block belongs to. We already know the type, the location, that it is valid
102and/or corrupted, and how long ago that it was last modified. Knowing the owner
103of the block is important as it allows us to find other related metadata to
104determine the scope of the corruption. For example, if we have a extent btree
105object, we don't know what inode it belongs to and hence have to walk the entire
106filesystem to find the owner of the block. Worse, the corruption could mean that
107no owner can be found (i.e. it's an orphan block), and so without an owner field
108in the metadata we have no idea of the scope of the corruption. If we have an
109owner field in the metadata object, we can immediately do top down validation to
110determine the scope of the problem.
111112Different types of metadata have different owner identifiers. For example,
113directory, attribute and extent tree blocks are all owned by an inode, whilst
114freespace btree blocks are owned by an allocation group. Hence the size and
115contents of the owner field are determined by the type of metadata object we are
116looking at. The owner information can also identify misplaced writes (e.g.
117freespace btree block written to the wrong AG).
118119Self describing metadata also needs to contain some indication of when it was
120written to the filesystem. One of the key information points when doing forensic
121analysis is how recently the block was modified. Correlation of set of corrupted
122metadata blocks based on modification times is important as it can indicate
123whether the corruptions are related, whether there's been multiple corruption
124events that lead to the eventual failure, and even whether there are corruptions
125present that the run-time verification is not detecting.
126127For example, we can determine whether a metadata object is supposed to be free
128space or still allocated if it is still referenced by its owner by looking at
129when the free space btree block that contains the block was last written
130compared to when the metadata object itself was last written. If the free space
131block is more recent than the object and the object's owner, then there is a
132very good chance that the block should have been removed from the owner.
133134To provide this "written timestamp", each metadata block gets the Log Sequence
135Number (LSN) of the most recent transaction it was modified on written into it.
136This number will always increase over the life of the filesystem, and the only
137thing that resets it is running xfs_repair on the filesystem. Further, by use of
138the LSN we can tell if the corrupted metadata all belonged to the same log
139checkpoint and hence have some idea of how much modification occurred between
140the first and last instance of corrupt metadata on disk and, further, how much
141modification occurred between the corruption being written and when it was
142detected.
143144Runtime Validation
145------------------
146147Validation of self-describing metadata takes place at runtime in two places:
148149 - immediately after a successful read from disk
150 - immediately prior to write IO submission
151152The verification is completely stateless - it is done independently of the
153modification process, and seeks only to check that the metadata is what it says
154it is and that the metadata fields are within bounds and internally consistent.
155As such, we cannot catch all types of corruption that can occur within a block
156as there may be certain limitations that operational state enforces of the
157metadata, or there may be corruption of interblock relationships (e.g. corrupted
158sibling pointer lists). Hence we still need stateful checking in the main code
159body, but in general most of the per-field validation is handled by the
160verifiers.
161162For read verification, the caller needs to specify the expected type of metadata
163that it should see, and the IO completion process verifies that the metadata
164object matches what was expected. If the verification process fails, then it
165marks the object being read as EFSCORRUPTED. The caller needs to catch this
166error (same as for IO errors), and if it needs to take special action due to a
167verification error it can do so by catching the EFSCORRUPTED error value. If we
168need more discrimination of error type at higher levels, we can define new
169error numbers for different errors as necessary.
170171The first step in read verification is checking the magic number and determining
172whether CRC validating is necessary. If it is, the CRC32c is calculated and
173compared against the value stored in the object itself. Once this is validated,
174further checks are made against the location information, followed by extensive
175object specific metadata validation. If any of these checks fail, then the
176buffer is considered corrupt and the EFSCORRUPTED error is set appropriately.
177178Write verification is the opposite of the read verification - first the object
179is extensively verified and if it is OK we then update the LSN from the last
180modification made to the object, After this, we calculate the CRC and insert it
181into the object. Once this is done the write IO is allowed to continue. If any
182error occurs during this process, the buffer is again marked with a EFSCORRUPTED
183error for the higher layers to catch.
184185Structures
186----------
187188A typical on-disk structure needs to contain the following information:
189190struct xfs_ondisk_hdr {
191 __be32 magic; /* magic number */
192 __be32 crc; /* CRC, not logged */
193 uuid_t uuid; /* filesystem identifier */
194 __be64 owner; /* parent object */
195 __be64 blkno; /* location on disk */
196 __be64 lsn; /* last modification in log, not logged */
197};
198199Depending on the metadata, this information may be part of a header structure
200separate to the metadata contents, or may be distributed through an existing
201structure. The latter occurs with metadata that already contains some of this
202information, such as the superblock and AG headers.
203204Other metadata may have different formats for the information, but the same
205level of information is generally provided. For example:
206207 - short btree blocks have a 32 bit owner (ag number) and a 32 bit block
208 number for location. The two of these combined provide the same
209 information as @owner and @blkno in eh above structure, but using 8
210 bytes less space on disk.
211212 - directory/attribute node blocks have a 16 bit magic number, and the
213 header that contains the magic number has other information in it as
214 well. hence the additional metadata headers change the overall format
215 of the metadata.
216217A typical buffer read verifier is structured as follows:
218219#define XFS_FOO_CRC_OFF offsetof(struct xfs_ondisk_hdr, crc)
220221static void
222xfs_foo_read_verify(
223 struct xfs_buf *bp)
224{
225 struct xfs_mount *mp = bp->b_target->bt_mount;
226227 if ((xfs_sb_version_hascrc(&mp->m_sb) &&
228 !xfs_verify_cksum(bp->b_addr, BBTOB(bp->b_length),
229 XFS_FOO_CRC_OFF)) ||
230 !xfs_foo_verify(bp)) {
231 XFS_CORRUPTION_ERROR(__func__, XFS_ERRLEVEL_LOW, mp, bp->b_addr);
232 xfs_buf_ioerror(bp, EFSCORRUPTED);
233 }
234}
235236The code ensures that the CRC is only checked if the filesystem has CRCs enabled
237by checking the superblock of the feature bit, and then if the CRC verifies OK
238(or is not needed) it verifies the actual contents of the block.
239240The verifier function will take a couple of different forms, depending on
241whether the magic number can be used to determine the format of the block. In
242the case it can't, the code is structured as follows:
243244static bool
245xfs_foo_verify(
246 struct xfs_buf *bp)
247{
248 struct xfs_mount *mp = bp->b_target->bt_mount;
249 struct xfs_ondisk_hdr *hdr = bp->b_addr;
250251 if (hdr->magic != cpu_to_be32(XFS_FOO_MAGIC))
252 return false;
253254 if (!xfs_sb_version_hascrc(&mp->m_sb)) {
255 if (!uuid_equal(&hdr->uuid, &mp->m_sb.sb_uuid))
256 return false;
257 if (bp->b_bn != be64_to_cpu(hdr->blkno))
258 return false;
259 if (hdr->owner == 0)
260 return false;
261 }
262263 /* object specific verification checks here */
264265 return true;
266}
267268If there are different magic numbers for the different formats, the verifier
269will look like:
270271static bool
272xfs_foo_verify(
273 struct xfs_buf *bp)
274{
275 struct xfs_mount *mp = bp->b_target->bt_mount;
276 struct xfs_ondisk_hdr *hdr = bp->b_addr;
277278 if (hdr->magic == cpu_to_be32(XFS_FOO_CRC_MAGIC)) {
279 if (!uuid_equal(&hdr->uuid, &mp->m_sb.sb_uuid))
280 return false;
281 if (bp->b_bn != be64_to_cpu(hdr->blkno))
282 return false;
283 if (hdr->owner == 0)
284 return false;
285 } else if (hdr->magic != cpu_to_be32(XFS_FOO_MAGIC))
286 return false;
287288 /* object specific verification checks here */
289290 return true;
291}
292293Write verifiers are very similar to the read verifiers, they just do things in
294the opposite order to the read verifiers. A typical write verifier:
295296static void
297xfs_foo_write_verify(
298 struct xfs_buf *bp)
299{
300 struct xfs_mount *mp = bp->b_target->bt_mount;
301 struct xfs_buf_log_item *bip = bp->b_fspriv;
302303 if (!xfs_foo_verify(bp)) {
304 XFS_CORRUPTION_ERROR(__func__, XFS_ERRLEVEL_LOW, mp, bp->b_addr);
305 xfs_buf_ioerror(bp, EFSCORRUPTED);
306 return;
307 }
308309 if (!xfs_sb_version_hascrc(&mp->m_sb))
310 return;
311312313 if (bip) {
314 struct xfs_ondisk_hdr *hdr = bp->b_addr;
315 hdr->lsn = cpu_to_be64(bip->bli_item.li_lsn);
316 }
317 xfs_update_cksum(bp->b_addr, BBTOB(bp->b_length), XFS_FOO_CRC_OFF);
318}
319320This will verify the internal structure of the metadata before we go any
321further, detecting corruptions that have occurred as the metadata has been
322modified in memory. If the metadata verifies OK, and CRCs are enabled, we then
323update the LSN field (when it was last modified) and calculate the CRC on the
324metadata. Once this is done, we can issue the IO.
325326Inodes and Dquots
327-----------------
328329Inodes and dquots are special snowflakes. They have per-object CRC and
330self-identifiers, but they are packed so that there are multiple objects per
331buffer. Hence we do not use per-buffer verifiers to do the work of per-object
332verification and CRC calculations. The per-buffer verifiers simply perform basic
333identification of the buffer - that they contain inodes or dquots, and that
334there are magic numbers in all the expected spots. All further CRC and
335verification checks are done when each inode is read from or written back to the
336buffer.
337338The structure of the verifiers and the identifiers checks is very similar to the
339buffer code described above. The only difference is where they are called. For
340example, inode read verification is done in xfs_iread() when the inode is first
341read out of the buffer and the struct xfs_inode is instantiated. The inode is
342already extensively verified during writeback in xfs_iflush_int, so the only
343addition here is to add the LSN and CRC to the inode as it is copied back into
344the buffer.
345346XXX: inode unlinked list modification doesn't recalculate the inode CRC! None of
347the unlinked list modifications check or update CRCs, neither during unlink nor
348log recovery. So, it's gone unnoticed until now. This won't matter immediately -
349repair will probably complain about it - but it needs to be fixed.